Patent GB-1126831-A (GB)
Title: Magnetic tape transducing system
IPC: G11B31/00
Filed: 1966-04-29
Abstract: 1,126,831. Automatic speed control. MINNESOTA MINING & M.F.G. CO. April 29, 1966 [April 29, 1965], No.19058/66. Heading G3R. [Also in Division G5] In a system for the uninterrupted recording and playback of data on magnetic tapes, a subsequently started tape 12 is phase-locked with a running tape 12a, which has reached its full capacity, by controlling the motion of the tape 12 during the take-over period. Tachometer generator 15 produces an output signal the frequency of which indicates the speed of motor 14 which drives tape 12, a frequency discriminator 17 having zero output when the motor is at its rated speed. Summing network 19 receives the signal from discriminator 17 and also a signal from phase comparator 21 which compares signals from reference oscillator 36 with signals from transducer 24 monitoring a sync. track on the tape 12. The sync. track is recorded when transducer 35 is connected with oscillator 36, a transducer 34 providing parallel data tracks. The advance of the tape 12a, is controlled in a similar manner. When the amount of tape 12a decreases to a certain value, tape meter 37 responds to start motor 14 which accelerates to rated speed. At this speed, discriminator 17 enables gates 32, 33, to connect data source 10 to transducer 34, and to connect transducer 35 to oscillator 36. The output from phase comparator 21 over-rides the other signals fed to summer 19 to adjust the speed of motor 14 until the error signal becomes zero. At the time that the motor 14 is started, switch 39 is actuated to couple oscillator 40, associated with sweep generator 42, to transducer 35a, to record a further sync. signal on tape 12a, the output frequency of the oscillator 40 beginning to rise from 60 kc in a sawtooth manner as soon as the motor 14 reaches rated speed. A similar signal is simultaneously recorded on tape 12. Band pass filter 26 rejects these further sync. signals when the motor 14 is under normal position-control action provided by comparator 21. At the end of a cycle of generator 42, switch 39 is opened and tape 12a is stopped. During the play-back mode, as the end of tape 12a approaches, the 60 kc signal appears in transducer 24a and band pass filter 55 produces an output to start motor 14. The transducer 24 picks up the sweep frequency signal on tape 12, and the output of band pass filter 56 is passed to balanced modulator 58 and filter 59 whose output is the carrier signal for balanced modulator 61. The modulator signal for the latter is provided by filter 55. If the instantaneous outputs of filters 55, 56, are different, the output of filter 63 differs from 200 kc, and the speed of tape 12 is altered to restore the balance by supplying the output signal to comparator 21. Phase detector 67 compares the output of signal source 36 with that of filter 63, the error signal re-setting flip-flop 66 to open gate 65 whereby the output of filter 63 is utilized for fine control of unit 30 during the phase-locking mode. As soon as the output of detector 67 is zero for a predetermined period, gate 65 is blocked, but gate 71 is opened to permit the signal from reference source 36 to take over control. The data flow is then shifted from transducer 51a to transducer 51. In a modification, wherein the two tape systems use the same reference signal source during the transition period for recording as well as playback, the outputs of filters 55, 56, are passed to a frequency comparator having a D.C. output whose sign indicates whether the tape 12 lags or leads the tape 12a. The D.C. signal is applied to an oscillator providing a reference signal to the comparator 21. After the output of the frequency comparator has reached zero, comparator 21 is connected to transducer 24a.